Photochemical method for carbon isotopic enrichment

ABSTRACT

In the process of the invention an isotopic starting material comprising at least two isotopic forms of CF 3  I and particularly a mixture of carbon-12 and carbon-13 isotopic species of CF 3  I is selectively isotopically enriched by means of a laser-induced photochemical dissociation followed by chemical combination to form a compound, such as an ethylenically unsaturated compound other than CF 3  I. The chemical combination takes the form of recombination of dissociation fragments or the combination of the CF 3  radical with a scavenger compound. The separation is carried out by irradiating a gaseous mixture of the starting materials at a reduced pressure with laser radiation, until a significant enrichment in isotopic CF 3  I is achieved. The wavelength of the radiation is selected so as to selectively excite one of the CF 3  I isotopes, thereby causing the excited species to dissociate and form at least one chemical species other than CF 3  I. The resulting mixture is enriched in selectively unexcited isotopic CF 3  I which can be separated from the reaction product mixture by conventional techniques.

BACKGROUND OF THE INVENTION

There has been much recent interest in the development of variousmethods of isotopic separation or enrichment. Although much of thiseffort has been directed to the separation of uranium isotopes, pure orenriched isotopes of other elements are also desirable, for example, astracer materials for medical research and diagnosis, biological researchand environmental studies.

Multiphoton dissociation of molecules using intense infrared lasers hasbeen the object of extensive investigation during the past few years(see Mukamel et al, J. Chem. Phys. 65, 5204 (1976) and Dever et al, J.Am. Chem. Soc. 98, 5055 (1976) as well as the references cited therein).An important application of this technique is to the separation ofisotopically labeled molecules, as reviewed in several references(Mukamel et al and Dever et al supra; Walther "Atomic and MolecularSpectroscopy with Lasers", Topics in Applied Physics, Vol. 2, LaserSpectroscopy, (Walther, Ed.) Springer-Verlog, 1976; Laser Spectroscopy,Proceedings of the Second International Conference, Megeve June 23-27,1975 (Haroche et al, Ed.), Springer-Verlog, 1975; Letokhov et al, Sov.J. Quant. Electr. 6, 129,259 (1976); and Aldridge III et al"Experimental and Theoretical Studies of Laser Isotope Separation",Physics of Quantum Electronics, Vol. 4, Laser Photochemistry, TunableLasers, and Other Topics, Jacobs et al, Ed., Addison-Wesley, 1976).Enrichment in the isotopes of H, B, C, Si, Cl, S and Os has beenreported.

V.S. Letokhov, Physics Today, May 1977, pages 23-31, provides a reviewof the art including a tabular recitation of successful laser isotopeseparations including the separation of C¹² and C¹³ by multiphotondissociation of CCl₄. This reference also describes a pulsed TEA CO₂-He-N₂ laser which has been employed in laser isotope separations.

Another recent survey article of interest is R. N. Zare "LaserSeparation of Isotopes", Scientific American, Feb. 1977, Vol. 236, No.2, pages 86-98.

Multiphoton dissociation of CCl₃ F and CF₃ Cl has been reported by Deveret al supra. Dever et al used a focused CO₂ laser to obtain up to 1.6%conversion of the parent molecule per flash at about 60 torr ofpressure. No investigation of the isotopic selectivity was reported.

Lyman et al, J. Appl. Phys. 47 595 (1976) have described enrichingcarbon-13 by mutliphoton dissociation of CF₂ Cl₂ (Freon-12). The ¹³ C/¹²C ratio of the starting material was increased by a factor of 1.65 byselectively dissociating ¹² CF₂ Cl₂.

The invention described herein using CF₃ I offers several advantagesover the processes shown in the above two references. CF₃ I can bedissociated at relatively low intensities. An unfocused TEA CO₂ lasergives sufficient power so that a measurable fraction of startingmaterial can be dissociated in fewer than 100 shots at one torr in areasonable cell volume. In addition, very high isotope separationfactors may be achieved in CF₃ I. In excess of 15% of the molecules inthe beam can be dissociated per laser pulse at high intensities, andenrichment factors of nearly 600 have been obtained.

The interaction of low intensity CO₂ laser radiation with CF₃ I has beenreported by Jones et al, J. Mol. Spectr. 58 125 (1975) and Petersen etal, Opt. Commun. 17, 259 (1976).

Photochemical isotopic enrichment techniques are based on two mainphenomena. First, there is the fact that the wavelengths of spectrallines absorbed by a molecule depend somewhat on the isotopes present inthe molecule. Second, the rate of a chemical reaction is sometimesinfluenced by the state of excitation of the participating molecules. Inorder for photochemical isotopic enrichment to be possible with a givenstarting material, several conditions must be satisfied. First of all,the effect of isotopic content of the starting material on thewavelengths of one or more of its spectral lines must be large enough sothat one type of isotope-containing molecule could be preferentiallyexcited by absorbing laser radiation which would not excite the othertype of isotope-containing molecules. Secondly, a laser is needed whoseradiation happens to match in wavelength one of the isotope-dependentlines, or a laser that can be tuned to such a wavelength, and thespectral width of the laser radiation must be narrow enough to excitemolecules containing one of the isotopes and not the others. Thirdly,the recombination of the fragments selectively dissociated by the laserto form the original starting compound must be retarded or prevented.Fourthly, transfer of excitation from one molecule to another bycollisions, and "scrambling" of isotopes through collisions of reactionproducts with other species must be negligible. Both these latterfactors contribute to the overall selectivity of the process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 plots experimental results showing the selectivity of productformation as a function of pressure at a laser intensity at 5.5 MW/cm².

FIG. 2 plots experimental results showing the remaining fraction of CF₃I as a function of the number of laser pulses and cell temperature.

FIG. 3 plots experimental results showing the fractional dissociation inthe laser beam per pulse as a function of laser intensity.

FIG. 4 is a schematic representation of the apparatus employed in theExamples.

DESCRIPTION OF THE INVENTION

It is accordingly a primary object of the present invention to provide alaser-induced photochemical method for carbon isotopic enrichment in anisotopic starting material containing at least two isotopic forms ofcarbon and preferably a mixture of carbon-12 and carbon-13 isotopicspecies which is capable of favorably satisfying the above describedconditions.

Another object of the present invention is to provide a laser-inducedphotochemical method for carbon isotopic enrichment which permitsselectivity in the isotopic species to be enriched.

A further object of the present invention is to provide a laser-inducedphotochemical method for the selective enrichment of either carbon-12 orcarbon-13 isotopic content of a starting material containing each ofthese two isotopes.

Still another object of the present invention is to provide alaser-induced photochemical method for the selective enrichment ofcarbon isotopic starting material, wherein the reaction products formedduring the course of the process are relatively stable.

Still a further object of the present invention is to provide alaser-induced photochemical method for the selective enrichment ofcarbon isotopic species in a carbon containing isotopic startingmaterial, which results in appreciable yield of an isotopically enrichedmaterial.

The above and other objects are achieved in accordance with the methodof the present invention in which an isotopic starting materialcomprising CF₃ I is subjected to a selective multiphoton dissociationprocess to enrich an isotopic carbon species thereof by selectivelyexciting an undesired isotopic species in admixture therewith.

In the presently preferred mode of the invention, an isotopic startingmaterial comprising a mixture of carbon-12 and carbon-13 isotopicspecies of CF₃ I is selectively isotopically enriched in carbon-13 bymeans of a laser-induced photochemical dissociation of the carbon-12species, followed by chemical combination to form a compound other than¹² CF₃ I. The chemical combination takes the form of recombination ofdissociation fragments, or the combination of the ¹² CF₃ radical with ascavenger compound. The method is carried out by irradiating a gaseousmixture of the starting material at a reduced pressure with laserradiation at a wavelength between 9.317 microns and 9.282 microns untila significant enrichment in ¹³ CF₃ I is achieved. The wavelength of theradiation is selected so as to selectively excite and dissociate the ¹²CF₃ I to form a chemical species other than ¹² CF₃ I. The resultingmixture is enriched in selectively unexcited isotopic ¹³ CF₃ I which canbe separated from the reaction product mixture by conventionaltechniques.

The particular laser employed is not critical except that it mustproduce or be tunable to produce radiation which selectively excitesmolecules containing one of the isotopic CF₃ I molecules and has aspectral width narrow enough so that it does not excite the isotopic CF₃I molecules sought to be enriched. In addition, the laser must havesufficient intensity to cause dissociation of isotopic CF₃ I to CF₃. +I. Preferably, the laser employed in the process of this inventionshould provide an intensity of at least about 1 to 2 MW/cm² (or at leastan energy of about 0.12 to about 0.24 Joules/cm²) and preferably anintensity of at least about 3.5 MW/cm² (0.42 Joules/cm²). Mostpreferably, a laser intensity of at least about 5 MW/cm² (0.6Joules/cm²) is employed.

While a pulsed laser is exemplified, a continuous beam laser is alsooperable in the process of this invention.

The duration of irradiation should be sufficient to cause a measurableenrichment in the desired isotopic species. With the exemplified TEA CO₂laser, about 200 shots are required. Obviously, laser intensity andpower, and process temperature and pressure affect the time required.For any particular set of parameters, the necessary reaction duration isreadily determined.

In order to achieve significant isotopic enrichment, it is necessary toreduce or prevent scrambling. In order to achieve this, the number ofcollisions during dissociation should be kept as low as possible, andpreferably the number of dissociations per unit time should be greaterthan the number of collisions per unit time. One method of reducingscrambling is by the use of high intensity, short duration pulsedradiation. For example, laser radiation at an intensity of 10 MW/cm²,with a duration of 10 nsec, will provide selectivity at relatively highpartially pressures. However in the presently preferred embodiment, inorder to reduce scrambling, the pressure, or at least the partialpressure of CF₃ I, in the reaction zone should be less than about 1torr, preferably this pressure should be less than about 0.5 torr. Inpractice, a pressure of about 0.1 torr has been found very useful.

While the dissociation reaction can occur over a wide range oftemperatures, it is preferred that the reaction mixture be maintainedunder conditions such that at the particular temperature and pressuremolecular I₂ condenses from the reaction mixture and/or has a lowpartial pressure so that the reaction CF₃.+I₂ →CF₃ I+I is prevented orretarded, thus increasing the efficiency of the enhancement process. Ithas been found that cooling at least a portion of the reaction mixtureat a pressure of less than 1 torr to about -50° C. and preferably toabout -80° C. or less causes I₂ to condense upon the walls of thereaction zone. This is evidenced by the fact that after a desired degreeof reaction when the gaseous reaction mixture is removed from thereaction zone, the gaseous reaction mixture contains little or no I₂,but a coating of I₂ remains behind on the cooled walls of the reactionzone. Cooling may be achieved in any practical manner, such as by theuse of a jacket about the side walls of the reaction zone or by thecooling of side arms extending from or fingers extending into thereaction vessel.

For a better understanding of the selectivity of this process, thefollowing parameters (¹³ CF₃ I enrichment) are of interest: Where β_(r)is the ratio of reactant isotope abundances before and afterirradiation:

    β.sub.r =[n.sub.13 /n.sub.12 ]/[n.sub.13 (0)/n.sub.12 (0)](1)

where n₁₃ (n₁₂) is the number density of ¹³ CF₃ I (¹² CF₃ I) moleculesand N_(i) (0) refers to the initial number of the i^(th) species. Thelaser preferentially dissociated ¹² CF₃ I so that β_(r) increases as thebulk dissociation proceeds.

In a similar fashion, β_(p) is defined as the isotope ratio in theproducts P₁₂ and P₁₃, compared to the ratio expected for a nonselectiveprocess:

    β.sub.p =[P.sub.12 /P.sub.13 ]/[n.sub.12 (0)/n.sub.13 (0)](2)

Both β_(r) and β_(p) increases with increasing selectivity.

The parameters β_(p) and β_(r), whose values depend on the amount oflaser energy applied, e.g., the number of pulses from a pulsed laser,are useful macroscopic indicators of the selectivity. The microscopicinformation concerning the selectivity is contained in the parameter α,defined as follows: In the case where a pulsed laser is employed, aftereach laser pulse, small increments dn₁₂ and dn₁₃ of n₁₂ and n₁₃ areconverted to products P₁₂ and P₁₃, respectively. The isotope ratio inthis increment of products are given by dn₁₂ /dn₁₃. Since a completelynonselective process will give an incremental isotope ratio equal to thecurrent value of n₁₂ /n₁₃, the macroscopic selectivity is measured bythe parameter

    α=[dn.sub.12 /dn.sub.13 ][/n.sub.12 /n.sub.13 ]      (3)

If ¹² CF₃ I molecules are preferentially dissociated, than α will belarger than one.

If f is defined to be the fraction of starting material remaining afterirradiation of the sample with several pulses,

    f=[n.sub.12 +n.sub.13 ]/[n.sub.12 (0)+n.sub.13 (0)]≈n.sub.12 /n.sub.13 (0)                                             (4)

where the approximate holds for n₁₂ >>n₁₃. Assuming that n₁₂ >>n₁₃ andthat α is constant during the course of the photolysis, the quantitiesof f, α and β_(r) are related by the equation ##EQU1## Combination ofEqns (2) and (3) and integration from f'=1 to f'=f to obtain β_(p)yields: ##EQU2##

In principle, α may be calculated from β_(r) and f, β_(p) and and f, orfrom β_(p) and β_(r).

Finally, to determine how f varies with N, the number of laser pulses,it is assumed that the fraction of CF₃ I dissociated in the beam perpulse, Δf is constant throughout the irradiation. Then, for a cell ofvolume V_(c) and a homogeneously irradiated volume V_(irr), Δf isrelated to f by

    f=(1-rΔf).sup.N                                      (7)

where r=V_(irr) /V_(c) and N is the total number of laser pulses. (Withreference to the above theoretical discussion, see Lyman et al, J. Appl.Phys. 41, 595 (1976).)

Examining the kinetics of the process of the invention, it has beenshown that the CF₃ and I radicals formed by multiphoton dissociation ofCF₃ I recombine to yield CF₃ I, C₂ F₆ and I₂. These are only speciesobserved in either infrared or mass spectra of samples irradiated atintensities below 25 MW/cm². In particular, CF₂ I₂ and other productswhich arise from breaking a C-F bond are not observed. There is proposedbelow a simplified kinetic scheme for the purpose of discussing majorfeatures in the results. This scheme is sufficient for that descriptionbut is by no means complete. ##EQU3## In these equations p and q areintegral numbers of photons totalling enough energy to break the C-Ibond (ΔH₂₉₈ ° = +55 kcal/mole; p + q ≳ 18 photons). Equations (8) and(9) may actually consist of several individual steps.

Dissociation is followed by recombination of radicals to yield productsor reactants:

    CF.sub.3 + CF.sub.3 → C.sub.2 F.sub.6               (11)

    cf.sub.3 + i → cf.sub.3 i                           (12)

    i + i (+m) → i.sub.2 (+m)                           (13)

two other radical reactions are also of importance:

    CF.sub.3 + I.sub.2 → CF.sub.3 I + I                 (14)

    .sup.12 cf.sub.3 + .sup.13 cf.sub.3 i ⃡ .sup.12 cf.sub.3 i + .sup.13 cf.sub.3                                          (15)

in an alternative embodiment, the starting isotopic CF₃ I is admixedwith an ethylinically unsaturated organic compound reactive with theCF₃. radical or isotopically selectively excited CF₃ I through theunsaturation, and which has an appreciable vapor pressure at thepressure and temperature employed in the reaction zone and which also isnot disproportionated by the laser beam employed. The presentlypreferred ethylinically unsaturated compound is ethylene. In a reactionmixture where the unsaturated compound is present, the followingreactions are believed to occur:

    *CF.sub.3 I → *CF.sub.3. + I

    *cf.sub.3. + ch.sub.2 ═ch.sub.2 → *cf.sub.3 ch.sub.2 ch.sub.2.

    cf.sub.3 ch.sub.2 ch.sub.2. + i → *cf.sub.3 ch.sub.2 ch.sub.2 i

there follows a number of examples which set forth specific embodimentsof the invention. These examples are to be considered illustrative,rather than limiting and all parts and percentages are by weight unlessotherwise specified. All temperatures are degrees centigrade unlessspecified.

EXAMPLES 1-24

The following examples show the enrichment of ¹³ CF₃ I.

CF₃ I dissociation was achieved employing CF₃ I with a natural ¹² C/¹³ Cdistribution, i.e. about 99/1. The laser used was a grating tuned CO₂TEA laser (Tachisto Corporation Model 215 laser head -- TachistoCorporation, Needham, Mass.) producing a maximum of 1 joule single lineoutput in 60 ns. fwhm. In several experiments, including Runs 13 and 14,a 30 cm. focal length sodium chloride lens was used to focus theradiation through polished NaCl windows into cylindrical Pyrex samplecells. The dimensions of the cells were adjusted to meet therequirements of individual experiments. Cell lengths ranged from 5 cm.to 30 cm., when focusing was used and from 5 cm. to 114 cm. when thelaser was used unfocused. Species indentification, concentrations, andisotope ratios were determined with a Perkin Elmer Model 521 gratinginfrared spectrometer and a Consolidated Engineering Corporation type21-103A mass spectrometer.

Laser power was measured with a Scientech Model 360001 laser powermeter. The pulse intensity was taken to be one-half of the measuredenergy per pulse in 60 nanoseconds over the mean irradiated area of asample. Beam areas were recorded on thermal sensing paper stock and werenot corrected for laser mode structure or external diffraction effects.An intracavity aperture near the output mirror was used to restrictlasing to low order transverse modes, and an external aperture wasgenerally used to reduce the beam area to 0.5 cm².

The following table summarizes twenty-four experiments using theequipment and conditions described:

                                      TABLE I                                     __________________________________________________________________________       CF.sub.3 I                                                                         Laser                                                                    Pressure                                                                           Intensity                  E.sub.abs                                                                          Eff.                                  Run                                                                              (torr)                                                                             (MW/cm.sup.2)                                                                       β.sub.r                                                                     β.sub.p                                                                     f.sup.b                                                                           r.sup.c                                                                           Δf                                                                          α                                                                          (hγ/mol)                                                                     (%)                                                                              N                                  __________________________________________________________________________       0.14.sup.d                                                                         5.5   1.7                                                                              >13                                                                              0.77                                                                              0.86                                                                              <.001                                                                             >13        400                                2  0.15 5.5   1.5                                                                              > 8                                                                              0.75                                                                              0.86                                                                              <.001                                                                             > 8        400                                3  0.32 5.5   1.0                                                                                4                                                                              0.95                                                                              0.86                                                                              <.001                                                                               4        170                                4  0.50 5.5   1.0                                                                                2                                                                              0.95                                                                              0.86                                                                              <.001                                                                               2        100                                5  0.60 5.5      1.9    0.86    1.9        200                                6  0.67 5.5   1.0                                                                              1.7                                                                              0.94                                                                              0.86                                                                              <.001                                                                             1.7        170                                7  0.83 5.5   1.0                                                                              1.3.sup.3                                                                        0.92                                                                              0.86                                                                              <.001                                                                             1.3        170                                8  0.95 5.5      1.2.sup.3                                                                            0.86    1.2        100                                9  1.00 5.5   1.0                                                                              1.1.sup.2                                                                        0.92                                                                              0.86                                                                              .001                                                                              1.1        100                                10 1.20 5.5      1.1.sup.1                                                                            0.86    1.1        100                                11 1.50 5.5      1.0.sup.8                                                                            0.86    1.1        100                                12 5.00 5.5   1.0                                                                              1.0.sup.2                                                                        0.73                                                                              0.86                                                                              .004                                                                              1.0        100                                13 0.15.sup.e                                                                         25(max)                                                                             180                                                                              5.3                                                                              0.02                                                                              0.078                                                                             .05 9.8        1000                               14 0.10.sup.e                                                                         25(max)                                                                             590                                                                              6.4                                                                              0.015                                                                             0.078                                                                             .03 12         2000                               15 0.10.sup.e                                                                         3.5   8.1                                                                              >25                                                                              0.20                                                                              0.414                                                                             .004                                                                              >41        1080                               16 0.50 0.8         1.  .077                                                                              0      2..sup.6                                                                           0  200                                17 0.50 1.5         0.99                                                                              .077                                                                              .001   4..sup.7                                                                           0.3                                                                              200                                18 0.50 3.3         0.90                                                                              .077                                                                              .007   8..sup.1                                                                           1.5                                                                              200                                19 0.50 5.2         0.56                                                                              .077                                                                              .038   11..sup.2                                                                          6.1                                                                              200                                20 0.50 6.1         0.65                                                                              .042                                                                              .051   12..sup.3                                                                          7.5                                                                              200                                21 0.50 7.0   1.7                                                                              4.4                                                                              0.45                                                                              .077                                                                              .052                                                                              5.7                                                                              13..sup.2                                                                          7.1                                                                              200                                22 0.50 10    1.6                                                                              4.7                                                                              0.63                                                                              .011                                                                              .084                                                                              5.6                                                                              16..sup.6                                                                          9.1                                                                              500                                23 0.50 13    3.2                                                                              4.0                                                                              0.33                                                                              .042                                                                              .13 5.6        200                                24 0.50 16    1.9                                                                              3.6                                                                              0.59                                                                              .007                                                                              .15 4.3        500                                __________________________________________________________________________     .sup.a For dissociation with the R(14) line of the 9.6 μ CO.sub.2 lase     transition.                                                                   .sup.b f=(.sup.12 CF.sub.3 I + .sup.13 CF.sub.3 I)/(.sup.12 CF.sub.3 I +      .sup.13 CF.sub.3 I) initial                                                   .sup.c r=V.sub.irr /V                                                         .sup.d MeOH-LN.sub.2 slush on side arm                                        .sup.e -80° C dry ice jacket on cell                              

Attention is directed to FIGS. 1-3 which plot experimental results. FIG.1 plots the selectivity of product formation as a function of pressureat a laser intensity of 5.5 MW/cm². FIG. 2 plots the remaining fractionof CF₃ I as a function of the number of laser pulses and celltemperature: T=298K (circles) and T=-80° C. (triangles). The dashed lineis the prediction of equation 7, above. FIG. 3 plots the fractionaldissociation in the beam per pulse, Δf, as a function of laserintensity. The pressure of CF₃ I was 0.5 torr.

EXAMPLE 25

Using the apparatus described in the previous examples, as schematicallyrepresented in FIG. 4 where the TEA CO₂ laser 1, was tuned by thegrating 2 to the R(14) line of the 9.6μCO₂ laser band, with thetransverse mode of beam 3, restricted by an intracavity aperture 4, toform beam 5, which emerged from the cavity through 50% mirror 6, andpassed in turn through the external aperture 7 to form beam 8, which wasfocused by lens 9 to form beam 10 which provides a peak intensity of 25MW/cm² at the center point 13 of a 30 cm cell 12 with sodium chloridewindows 11; when the cell was charged through feed tube 14 withnaturally occurring CF₃ I(¹³ C/¹² C≈1/99) at 0.10 torr, maintained at-80° C. with a Dry Ice jacket 15 and irradiated with 2000 pulses(0.5H_(z)) (60 nsec. fwhm.), mass spectral analysis showed a 590 foldenrichment of ¹³ CF₃ I.

While the above description of the invention has been primarilyconcerned with the enrichment of ¹³ CF₃ I, the invention is equallyapplicable to the enhancement of ¹² CF₃ I by the use of a laser beamhaving a wavelength between about 8.3 microns and about 8.6 microns,preferably about 8.5 microns, which selectively excites ¹³ CF₃ I in themanner described above to provide enrichment in ¹² CF₃ I.

In a similar fashion it is to be noted that the selective excitation of¹² CF₃ I provides enrichment of ¹⁴ CF₃ I. If desired, the laserwavelength may be chosen so that ¹⁴ CF₃ I can be enriched relative toany ¹³ CF₃ I in admixture therewith.

While the invention has been exemplified above in what is now consideredits best embodiments, it is to be understood that, within the scope ofthe above disclosure, the equipment and process conditions can be variedfrom those specifically described while still obtaining results withinthe scope of this invention.

We claim:
 1. A photochemical method for selective carbon isotopicenrichment in a starting material comprising a gaseous mixture of atleast two isotopic CF₃ I species which comprises irradiating a gaseousisotopic mixture comprising said CF₃ I at a pressure below about 10 torrin a reaction zone with laser radiation having an energy of at leastabout 0.1 joule/cm² and having a wavelength selectively coinciding withthe absorption band of one but not the other of said CF₃ I species,thereby selectively exciting and disproportionating said one species andcausing it to form a reaction product other than said one species toprovide relative enrichment of said other species; continuing saidirradiation for a period of time to cause significant relativeenrichment of said other species; and recovering from the gaseous phasein said reaction zone unreactive isotopic starting material enriched insaid other species.
 2. The method of claim 1 wherein the partialpressure of the isotopic CF₃ I starting material in said gaseous mixtureis below about 1 torr.
 3. The method of claim 2 where said partialpressure is about 0.1 torr.
 4. The method, as in claim 2, where thetemperature is sufficiently low, at the particular pressure employed tocause I₂ to condense from the irradiated gaseous mixture.
 5. The method,as in claim 3, where the temperature of at least a portion of thereaction zone is maintained at below about -70° C.
 6. The method, as inclaim 5, where the laser radiation energy is at least about 0.42joule/cm².
 7. The method of claim 6 wherein the partial pressure of theisotopic CF₃ I starting material in said gaseous mixture is below about1 torr.
 8. The pressure of claim 7 where said partial pressure is about0.1 torr.
 9. The method, as in claim 7, where the temperature issufficiently low, at the particular pressure employed to cause I₂ tocondense from the irradiated gaseous mixture.
 10. The method, as inclaim 8, where the temperature of at least a portion of the reactionzone is maintained at below about -70° C.
 11. The method, as in claim 6,where ¹² CF₃ I is said one excited species and ¹³ CF₃ I is said enrichedother species.
 12. The method of claim 11 wherein the partial pressureof the isotopic CF₃ I starting material in said gaseous mixture is belowabout 1 torr.
 13. The method of claim 12 where said partial pressure isabout 0.1 torr.
 14. The method, as in claim 12, where the temperature issufficiently low, at the particular pressure employed to cause I₂ tocondense from the irradiated gaseous mixture.
 15. The method, as inclaim 13, where the temperature of at least a portion of the reactionzone is maintained at below about -70° C.
 16. The method, as in claim10, where the laser radiation has a wavelength between about 9.317microns and about 9.282 microns.
 17. The method, as in claim 11, wherethe laser radiation is the R(14) line of a 9.6μ CO₂ laser band.
 18. Aphotochemical method for the selective carbon isotopic enrichment in astarting material comprising a gaseous mixture of at least two isotopicCF₃ I species which comprises irradiating a gaseous isotopic mixturecomprising said CF₃ I and an ethylinically unsaturated compound relativewith a CF₃. radical or isotopically selectively excited CF₃ I and inertto the radiation applied, at a partial pressure below about 10 torr, ina reaction zone with laser radiation having an intensity of at leastabout 1 MW/cm² and having a wavelength selectively coinciding with theabsorption band of one but not the other of said CF₃ I species, therebyselectively exciting said one species and causing CF₃ - andI-substituents to be added across the double bond of said ethylinicallyunsaturated compound to form a reaction product to provide relativeenrichment of said other species; continuing said irradiation for aperiod of time to cause significant relative enrichment of said otherspecies; and recovery from the gaseous phase in said reaction zoneunreactive isotopic starting material enriched in said other species.19. The method, as in claim 18, where the excited species is ¹² CF₂ Iand the unsaturated compound is ethylene and the reaction product is ¹²CF₃ CH₂ CH₂ I.
 20. The method as in claim 18 where the partial pressureis below about 1.5 torr.